You need to declare that your document may show up in the tree.
document_tree:
Acme\DemoBundle\Document\YourPage:
valid_children:
- Symfony\Cmf\Bundle\SimpleCmsBundle\Doctrine\Phpcr\Page
Symfony\Cmf\Bundle\SimpleCmsBundle\Doctrine\Phpcr\Page:
valid_children:
- Acme\DemoBundle\Document\YourPage